The present invention generally relates to methods of producing semiconductor-on-insulator structures and semiconductor devices having the semiconductor-on-insulator structure, and more particularly to a method of producing a semiconductor-on-insulator structure such as a silicon-on-insulator (SOI) structure and a semiconductor device having such a semiconductor-on-insulator structure.
The SOI technology was proposed as a method of forming an insulator layer between two single crystal semiconductor layers for the purpose of producing high-speed elements and semiconductor devices which are uneasily affected by alpha-rays. As methods of producing the SOI structure, there are the silicon on sapphire (SOS) technique, the laser melt technique, the wafer bonding technique and the like. However, according to the SOS technique and the laser melt technique, it is difficult to form a perfect single crystal layer on the insulator layer. For this reason, there is much attention on the wafer bonding technique.
FIGS. 1A and 1B are diagrams for explaining a conventional bonding technique. As shown in FIG. 1A, the surface of one of a base substrate 40 and an active substrate 41 is covered by a silicon dioxide (SiO.sub.2) layer 42. In this example, the SiO.sub.2 layer 42 covers the base substrate 40. The active substrate 41 is bonded on the base substrate 40 having the SiO.sub.2 layer 42 as indicated by an arrow. Thereafter, the active substrate 41 is subjected to lapping and polishing processes so as to remove a portion of the active substrate 41 indicated by a phantom line in FIG. 1B. As a result, the remaining active substrate 41 on the SiO.sub.2 layer 42 of the base substrate 40 has a thickness of approximately 5 microns.
When using the SOI structure shown in FIG. 1B to make a metal oxide semiconductor field effect transistor (MOSFET), a gate electrode g of the MOSFET is formed on the base substrate 40 via an insulator layer 43 as shown in FIG. 2. Then, two n-type regions are formed in the active substrate 41 to form a source s and a drain d of the MOSFET.
But molecules which lack oxygen such as SiO and Si.sub.2 O.sub.3 molecules exist within the SiO.sub.2 layer 42 which is provided between the base substrate 40 and the active substrate 41. For this reason, a positive interface state occurs at the SiO.sub.2 interface, and an inversion and a depletion state easily occur at the interface on the side of the active substrate 41. When such inversion and depletion state occur, an electron transition naturally occurs between the source s and the drain d of the MOSFET and causes an erroneous operation of the MOSFET.
In order to prevent the above described erroneous operation of the MOSFET, it is necessary to prevent the inversion and depletion state from occurring in the SiO.sub.2 layer 42.
FIG. 3 shows one conventional method of preventing the inversion and depletion state from occurring in the SiO.sub.2 layer 42. In FIG. 3, a negative voltage is applied to the base substrate 40 from a voltage source 44.
FIG. 4 shows another conventional method of preventing the inversion and depletion state from occurring in the SiO.sub.2 layer 42. In FIG. 4, a p-type impurity a such as boron (B) is injected into the interface on the side of the active substrate 41, so as to prevent the generation of negative charges in the active substrate 41.
However, according to the method shown in FIG. 3, there is a problem in that a control device for controlling the semiconductor device becomes bulky and complex because of the need to add the voltage source 44 for applying the negative voltage to the base substrate 40. For example, in the case of a complementary metal oxide semiconductor (CMOS) device, there is a need to provide two bias voltages for the p-channel n-channel transistors.
On the other hand, according to the method shown in FIG. 4, there is a problem in that it is difficult to adjust a threshold voltage of the MOSFET due to the injected impurity ions. In addition, there is also a problem in that the p-type impurity a diffuses into the active substrate 41 and varies the threshold voltage of the gate g when a thermal process is carried out to form elements on the active substrate 41. Hence, it is virtually impossible to make an active substrate which is sufficiently thin and does not contain the diffused p-type impurity a.
In order to eliminate the above described problems, a bonding method was proposed in a Japanese Laid-Open Patent Application No. 1-186612. According to this proposed method, the SOI structure is formed as shown in FIGS. 5A through 5E. The surface of an active substrate 51 is covered by a SiO.sub.2 layer 53 as shown in FIG. 5A. On the other hand, the surface of a base substrate 61 is covered by a SiO.sub.2 layer 63 as shown in FIG. 5B. The thickness of the SiO.sub.2 layer 63 is greater than that of the SiO.sub.2 layer 53. A negative fixed charge nfc is formed in the SiO.sub.2 layer 53 by injecting aluminum (Al) ions or the like as shown in FIG. 5C. Then, as shown in FIG. 5D, the active substrate 51 and the base substrate 61 are bonded together so that the SiO.sub.2 layer 63 of the base substrate 61 makes contact with the SiO.sub.2 layer 53 which has the negative fixed charge nfc. As a result, the positive charge within the SiO.sub.2 layer 53 is eliminated. The structure shown in FIG. 5D is then formed into the structure shown in FIG. 5E by polishing the top portion of the active substrate 51.
But according to this proposed method, the SiO.sub.2 layer 53 is formed on the active substrate 51 by a thermal oxidation, and the formed SiO.sub.2 layer 53 only has a thickness in the range of 0.4 micron to 1.0 micron. For this reason, when the Al ions are injected into the SiO.sub.2 layer 53 at a high energy, the Al ions easily penetrate the SiO.sub.2 layer 53 and reach the active substrate 51. When the Al ions are injected into the active substrate 51, there are problems in that the characteristics of elements such as transistors formed on the active substrate 51 change and the performance of the semiconductor device becomes poor. These problems become notable especially when the active substrate 51 is made thin since a large portion of the active substrate 51 is damaged by the Al ions which penetrated the SiO.sub.2 layer 53.
In order to prevent the Al ions from penetrating the SiO.sub.2 layer 53, it is possible to consider reducing the energy at which the Al ions are injected. To prevent the penetration of the Al ions, the energy must be reduced to 10 keV or less, but at such a small energy the injection coefficient becomes greatly reduced and it becomes difficult to adjust the ion injection quantity.